William Balfour admitted killing singer Jennifer Hudson's mother and brother to his girlfriend just hours after they were fatally shot, the woman testified Friday in what turned out to be a powerful day of testimony for the prosecution.

Appearing uncomfortable at times on the witness stand, Shonta Cathey told jurors in a steady voice that Balfour came to her West Side apartment shortly after noon on the day of the slayings and told her he had killed the mother and brother but that Hudson's 7-year-old nephew was safe.

"He said he went in (to the Hudson home) and the brother rushed him, so he shot him," Cathey testified. She said that Balfour told her Hudson's mother came down the stairs calling Jason's name, "and he shot her too."

"I was looking at him crazy, like bug-eyed … just staring at him," Cathey said. "He asked why was I looking at him like that (and said) I had nothing to worry about."

As Cathey recounted the conversation, Jennifer Hudson leaned forward and sniffled while her fiance wrapped his arm around her shoulder. Seated to her left, her sister, Julia Hudson, Balfour's former wife, wiped away tears.

Cathey said she was terrified for her safety and that of her two young children and agreed to serve as Balfour's alibi if police inquired about his whereabouts. She allowed him to hide out the rest of the day in her apartment, where she said he talked on his phone and watched television. Cathey said she walked to the corner store to buy beer.

"I was scared and wanted to do everything he wanted, so he wouldn't hurt us," she said.

Balfour, 30, is accused of killing Hudson's mother, Darnell Donerson; brother Jason Hudson; and nephew Julian King, on Oct. 24, 2008, in an act of spite against Hudson's sister. Balfour and Julia Hudson had been separated for nearly eight months at the time of the murders and she had recently begun dating someone else.

Cathey testified that she asked about "the little boy's" whereabouts after Balfour told her Donerson and Jason Hudson were dead. He replied that Julian "was outside" and there was nothing to worry about, she said. The boy was found three days later a couple miles from Cathey's home, shot to death in Jason Hudson's stolen SUV.

Police arrested Balfour at Cathey's apartment later that night, locating him through cellphone records. She agreed to let her home be searched and later went to the station for questioning. She testified that she initially lied to police, insisting that Balfour had been with her at 10 a.m. that day just as he had asked her to do.

"I was in love with William," she said. "And I didn't want him to get in any trouble."

Later, she gave a statement to detectives about Balfour's confession.

In other testimony, William Graham told jurors he was looking out the window of his West Side home on the day of the triple homicide and saw Balfour drive by in a white SUV. Graham, 71, said he recognized Balfour because he sometimes stayed with Cathey, who lived across the street.

Although there were no eyewitnesses to the murders and no direct evidence tying Balfour to the crime scene, prosecutors have presented numerous witnesses placing him near the Hudson home on the morning of the slayings. Others have testified they saw him with Jason Hudson's .45-caliber pistol, identified by prosecutors as the weapon used in the killings.

FBI Special Agent Nikki Skovran, a forensic cellphone analyst, testified that Balfour made three calls between 7:28 a.m. and 9:02 a.m. the day of the murders — and all three pinged from the cellular tower closest to the Hudson home.

Skovran acknowledged that the records cannot pinpoint an exact location, only a general vicinity.

Balfour's final call that morning was from Julia Hudson, who argued with her estranged husband about their car. Cell records show that when the call ended at 9:02 a.m., he was within about a mile radius of the Hudson home. Witnesses heard gunshots coming from the home a short time later, but they did not call police.

The phone then went off the network — meaning it was probably either turned off or had run out of batteries — for nearly four hours, Skovran said.